New emissions legislation in Europe and North America is driving the implementation of new exhaust aftertreatment systems, particularly for lean-burn technologies such as compression-ignition (diesel) engines, and stratified-charge spark-ignited engines (usually with direct injection) that are operating under lean and ultra-lean conditions. Lean-burn engines exhibit high levels of nitrogen oxide emissions (NOx), that are difficult to treat in oxygen-rich exhaust environments characteristic of lean-burn combustion. Exhaust aftertreatment technologies are currently being developed that treat NOx under these conditions.
One of these technologies includes a catalyst that facilitates the reactions of ammonia (NH3) with the exhaust nitrogen oxides (NOx) to produce nitrogen (N2) and water (H2O). This technology is referred to as Selective Catalytic Reduction (SCR). Ammonia is difficult to handle in its pure form in the automotive environment, therefore it is customary with these systems to use a liquid aqueous urea solution, typically at a 32% concentration of urea (CO(NH2)2). The solution is referred to as AUS-32, or diesel exhaust fluid (DEF), and is also known under its commercial name of AdBlue. The DEF is delivered to the hot exhaust stream and is transformed into ammonia in the exhaust after undergoing thermolysis, or thermal decomposition, into ammonia and isocyanic acid (HNCO). The isocyanic acid then undergoes a hydrolysis with the water present in the exhaust and is transformed into ammonia and carbon dioxide (CO2), the ammonia resulting from the thermolysis and the hydrolysis then undergoes a catalyzed reaction with the nitrogen oxides as described previously.
The delivery of the DEF solution to the exhaust involves precise metering of the DEF and proper preparation of the DEF to facilitate the later mixing of the ammonia in the exhaust stream. The delivery of the DEF into the exhaust is typically achieved using some type of injector.
The injector is calibrated during manufacturing to function correctly during the life of the vehicle such that a consistent amount of DEF is injected into the exhaust stream each time the injector is actuated. One approach to achieving the proper calibration is to use a calibration sleeve which is mounted in a specific position to properly position a return spring (which is part of a solenoid unit used for actuating the injector). However, when the vehicle is exposed to different environments and operating conditions, the DEF may freeze, and therefore expand. Injectors are typically expected to operate at temperatures between −40° C. to 160° C. DEF freezes at −11° C., which may occur in cold environments when the vehicle is not in use, and the volume of DEF increases by approximately 9% when frozen. Since the DEF in its liquid form is able to migrate around different parts of the injector, the expansion of the DEF during freezing may cause different components of the injector to shift and deform, or displace permanently, compromising the operation of the injector, and affecting performance. Some injectors incorporate the use of external devices to compensate for this expansion, which add cost and number of components.
Accordingly, there exists a need for an injector which is able to compensate for the increase in volume of frozen diesel exhaust fluid during certain conditions, and still function correctly once the frozen diesel exhaust fluid has returned to liquid form.